Direct hyposmotic stimulation of gastric acid secretion.

Gastric glands isolated from rabbit stomach were incubated in isosmotic medium or media made hyposmotic by 50-100 mOsm/kg. As indicated by radiolabeled aminopyrine accumulation, acid secretion was nearly 3 times greater in 200 mOsm/kg hyposmotic than in isosmotic medium after a 30-min incubation. The hyposmotic stimulation appeared within 2 min, peaked at 10-15 min and declined almost to the isosmotic control by 45 min. As estimated by the wet weight corrected for inulin extracellular space, the intracellular water of the glands also peaked at 15 min and returned to the isosmotic norm by 45 min. Hyposmotic stimulation of acid secretion directly involved the parietal cell, since parietal cells obtained from gastric glands were also stimulated. That the hyposmotic response was direct was indicated by omeprazole inhibition of aminopyrine accumulation in hyposmotic medium.

Histamine-mediated hyposmotic stimulation of gastric acid secretion.

Gastric glands incubated in hyposmotic medium (200 mOsm) accumulated aminopyrine, a measure of acid secretion, to the same extent as that of paired glands in isomotic medium containing histamine (10(-4) M). These maximal responses to hyposmolality and histamine were not additive. The hyposmotic response peaked earlier than the histamine response. Hyposmotic stimulation was nearly abolished by preincubation of the glands with metiamide and cimetidine, H-2 histamine antagonists. In the presence of histaminase, no hyposmotic stimulation occurred. The response to forskolin, a stimulant of adenylate cyclase, was equivalent in hyposmotic and isosmotic media. These results indicate that hyposmolality releases histamine from a paracrine cell in the gastric gland and that histamine binds to H-2 receptors on the parietal cell to initiate a cyclic AMP-mediated stimulation of acid secretion.

Milrinone produces a positive inotropy but is not inhibitory to gastric ion transport.

The cardiac drugs ouabain and milrinone are positive inotropic agents. Since ouabain has inhibitory effects on ion transport in the gastrointestinal tract associated with vasoconstriction and hypoxia, milrinone needed to be tested also. This report indicates that therapeutic levels of milrinone on either side of the isolated stomach wall of the guinea pig has no significant effects on active chloride ion transport or the electrical parameters of the tissue. To verify that milrinone was active in the heart at these same levels, contractility of the isolated heart of the guinea pig was measured. Milrinone significantly increased ventricular pressure and pressure development. Thus milrinone may be expected to exert its inotropic stimulation of the heart during heart failure without compromising gastrointestinal functions.

Effects of bile salts and prostaglandins on sodium transport in isolated rat gastric mucosa.

To determine whether prostaglandins may protect against bile salt inhibition of ion transport in the stomach, gastric mucosal tissue was isolated from the rat and mounted in flux chambers. Transport of Na+ was traced with radioisotopes in the absence of bile salts and then in the presence of conjugated taurocholate or unconjugated deoxycholate at low, intermediate and high mucosal concentrations (1, 5 and 15 mmol/1). At a high (7.40) or low (3.4) mucosal pH, only the unconjugated deoxycholate inhibited active Na+ transport from mucosa to submucosa with respect to untreated controls. Inhibition of Na+ transport was apparent at a low level of deoxycholate, which also inhibited the electrical potential difference. Intermediate and high levels of deoxycholate lowered the tissue resistance. When the tissues were exposed to mucosal prostaglandin E2 or its 16,16-dimethyl analogue before and during acidified taurocholate administration, Na+ transport was not changed significantly but the electrical resistance remained high. Thus, unconjugated bile salt is more potent than conjugated bile salt in inhibiting Na+ transport and breaking the gastric mucosal barrier, and prostaglandins may afford some small protection.

Intraluminal prostaglandin stimulation and bile salt inhibition of basal acid secretion in the instilled rat stomach.

To determine the effect of prostaglandin on acid secretion and the effectiveness of cytoprotection afforded by prostaglandins against bile salt damage, the stomachs of anesthetized rats were exteriorized and cannulated for determination of net acid output (H+) and electrical potential difference (PD). The stomachs were successively instilled with acid buffer, 5 mM and 15 mM sodium taurocholate (NaTC) with or without 3.3 X 10-6M 16, 16-dimethyl prostaglandin E2 (dmPGE2). With dmPGE2 present in the instillate, H+ was twice as large in the control period. Exposure to 5mM NaTC reduced H+ by 63% and abolished the stimulatory effect of dmPGE2. Subsequent exposure to 15mM NaTC depressed H+ further. In the absence of dmPGE2, NaTC reduced H+ to equivalent rates. The PD was unaffected by NaTC in the presence of dmPGE2 and lowered by 45% in the absence of dmPGE2. It may be concluded that 1) prostaglandins stimulate rather than inhibit basal H+ in vivo, and 2) prostaglandins provide weak protection against the ionic actions of conjugated bile salts instilled in the stomach.

Effects of caffeine on transport, metabolism and ultrastructure of isolated rat colon.

The effect of caffeine on the transport, metabolism and ultrastructure of the colon were determined. Segments of proximal colon were excised from the anesthetized rat and prepared for radioisotopic tracing of ion transport in the flux chambers or oxidative metabolism in an incubator. Other segments were fixed before or after caffeine administration for electron microscopy. The isolated rat colon actively transported both Na+ and Cl- in the absorptive direction, mucosa to serosa. Serosal addition of 10 mmol/l caffeine abolished the smaller Na+ transport but did not significantly affect the larger Cl- transport. The electrical potential difference and the short-circuit current rose accordingly. Although the oxidation of glucose was inhibited by 35%, caffeine had no significant effect on the oxidation of the fatty acid, butyric acid. Comparable metabolic responses were obtained using the isolated terminal ileum of the rat. Neither the height nor the density of the microvilli in the proximal colon were affected significantly by caffeine. It may be concluded that caffeine, unlike theophylline, effectively preserves the normal absorptive condition of the colon. Thus, caffeine may have actions other than inhibition of phosphodiesterase in the distal intestine.

Antiabsorptive effects of 16,16 dimethyl prostaglandin E2 in isolated rat colon.

Ulcerative colitis is distinguished by abundant prostaglandin E2 (PGE 2) in the stools and by severe diarrhea. To determine whether luminal PGE2 alters normal colonic absorption, NA+ and Cl-transport across isolated rat proximal colon were studied before and after 16,16 dimethyl PGE2 (dmPGE2) addition to flux chambers. Luminal administration of dmPGE2 significantly reduced the net mucosal to serosal fluxes of Na+ and Cl-. These antiabsorptive tive effects of dmPGE2 on NA+ and Cl- active transport were reflected by a reduced metabolic rate of colonic tissue slices incubated with dmPGE2. Addition of dmPGE2 significantly reduces oxidation of glucose by the colon. Structurally, dmPGE2 reduced the length of colonic mucosal microvilli, thereby decreasing absorptive surface area. These results suggest that PGE2 released into the colonic lumen of patients with ulcerative colitis exerts antiabsorptive effects on the colon and in this way contributes to the associated diarrhea.

Prostaglandin E2 stimulation of oxygen consumption in parietal cells and of H+ transport in gastric mucosa of the rat.

Since prostaglandin E2 is generally stimulatory to intestinal and nonparietal gastric secretions, possible stimulation of parietal gastric acid secretion was investigated. Oxygen consumption of parietal cells isolated from rat stomach was determined before and after addition of 16,16-dimethyl prostaglandin E2 (dmPGE2). At low concentration, dmPGE2 significantly stimulated oxygen consumption of parietal cells by 7%. Concomitantly, acid secretion rose. H+ transport in the isolated gastric mucosa of the rat was determined in the absence of electrochemical gradients to preclude H+ back-diffusion. Compared with control rates, H+ transport from submucosal to mucosal side was stimulated 51% by the same low concentration of dmPGE2. It is concluded that stimulation of gastric acid secretion by parietal cells appears to be a physiological function of prostaglandin E2 in the stomach. The results support the hypothesis that prostaglandins stimulate secretory responses throughout the gastrointestinal tract.

Hyperosmotic oxidation of glucose and decarboxylation of histidine in the gastric mucosa.

Paired slices of rat gastric mucosa were incubated with labeled glucose or histidine in isosomotic solution of 3-fold hyperosmotic solutions concentrated in NaCl, KCl, or ethanol. The rate of (1-14C)glucose oxidation to 14CO2 in isosmotic solution was reduced by 74% in hyperosmotic NaCl and by 28% in hyperosmotic KCl. The rate of (6-14C)glucose oxidation to 14CO2 in isosmotic solution was reduced by 64% in hyperosmotic NaCl and by 53% in hyperosmotic KCl. Reductions of glucose oxidation in hyperosmotic ethanol were not significant. The ratio of 14CO2 formed from (1-14C)glucose to that formed from (6-14C)glucose was not significantly changes by hyperosmotic NaCl or ethanol, but was significantly raised by hyperosmotic KCl. The rate of (carboxyl-14C)histidine decarboxylation in isosomotic solution was reduced significantly by 48% in hyperosmotic NaCl, by 30% in hyperosmotic KCl, and by 27% in hyperosmotic ethanol. We conclude that hyperosmotic solutions reduce glucose oxidation and histidine decarboxylation by rat gastric mucosa in the order of potency: NaCl greater than Kcl greater than or equal to ethanol. Thus hyperosmotic solutions inhibit the source of metabolic energy for stimulated acid secretion, the citric acid cycle, and the formation of the secretagogue histamine.